1. Field of the Invention
The present invention relates to the field of current sources.
2. Prior Art
Current sources are widely used in integrated circuits for such purposes as providing the desired biasing for various circuit elements within the circuit. A common approach in the design of such circuits is to provide one or more current references onchip, and then to mirror each current source from the reference to a plurality of nodes in the circuit in accordance with the circuit node requirements. The current sources themselves may have various characteristics, such as being substantially temperature independent, being proportional to absolute temperature (PTAT), being dependent on the power supply voltage, being substantially independent of power supply voltage, etc.
Mirroring currents in this manner minimizes the number of individual current references needed, allows current ratios to be set by relative transistor sizes, and maintains current ratios in spite of process variations. However, mirroring currents in this manner results in mirrored current sources which do not always maintain the desired high output impedance over a wide operating voltage range of the current source, particularly for low voltage drops across the transistors to which the current is mirrored, as may be required in low voltage circuits. Preferably the current sources should maintain a high output impedance for voltage drops across the current sources of as little as a single VCE.sub.(SAT) for bipolar transistors or V.sub.DS(sat) for MOS transistors. In the prior art, high output impedance current sources could be achieved using a cascode current source, but this increases the headroom needed for the operation of the current source.